The use of coiled tubing for various well treatment processes such as fracturing, acidizing and gravel packing is well-known. The advantages in the use of coiled tubing include relatively easy and quick entry into a well without the necessity of employing complex and costly apparatus such as a workover derrick and the insertion of a tubing string into the well which is made up of a plurality of short lengths of tubing and which must be individually joined together.
Typically, several thousand feet of tubing is coiled onto a large reel which is mounted on a truck or skid. A coiled tubing injector head, typically employing chain-track drive, is mounted axially above the wellhead and the coiled tubing is fed to the injector for insertion into the well. The coiled tubing is plastically deformed as it is payed out from the reel and over a gooseneck guide which positions the coiled tubing along the axis of the wellbore and the injector drive mechanism.
Tools used with coiled tubing generally comprise a long ridged element having a central bore which, when attached to the coiled tubing, allows fluid communication between the bore of the coiled tubing through the tool and outwardly through various valves and ports in the tool to the wellbore itself. The tools also typically include one or more packer elements which act to isolate certain portions of the wellbore from each other. Such tools may be of any length but, for instance, for treatment of a particular interval in the wellbore, the tool must incorporate packer elements which, when positioned in the wellbore, effectively straddle and isolate that portion of the wellbore from the remaining portions, both above and below the zone of interest. Obviously, if the interval to be treated is particularly long, the treatment tool must be similarly of great length. Thus, tools of seventy-five feet in length or longer are not uncommon.
The injection of such a long-length tool which cannot be plastically deformed in the manner of coiled tubing and which is typically of a larger diameter than the coiled tubing itself such that it will not pass through the injector drive mechanism presents some difficulty. In order to overcome this difficulty, it has been common prior practice to mount the tool in what is effectively an extension of the well casing above the wellhead and positioning the injector drive mechanism on top of this pressurized cylindrical enclosure. Obviously, this places the bulky and heavy injector drive mechanism at an extreme height above the wellhead when long tools are to be used. Such weight cannot be supported solely by the cylindrical tool conduit and, therefore, must be at least partially supported by a heavy-duty crane or derrick in position over the wellhead. Despite employing guy-wires to steady the positioning of the top-heavy elevated drive mechanism, the uneven and irregular lateral forces applied to the assembly by unreeling and bending of the coiled tubing make such positioning difficult at best and an extreme safety hazard at worst.
U.S. Pat. No. 4,091,867, attempts to overcome these deficiencies by mounting the injector essentially at or near the surface and directing the coiled tubing upwardly to a high-mounted gooseneck and thereby into axial alignment with the wellbore. In this arrangement, the tubing must pass through a pressurized conduit for its entire length of travel from the injector head over the gooseneck to the wellhead. In such an arrangement, the gooseneck is placed above the highest point of a tool to be injected and, therefore, especially with long-length tools, the pressurized conduit extending from the injector head to the wellhead must be of long length with its associated problems of weight, pressurization and potential sealing failure. Additionally, the plastic deformation of the coiled tubing over the gooseneck is conducted with the tubing being in compression which can shorten tubing life when compared to plastic deformation in tension as is accomplished when the injector head is axially mounted above the wellbore and draws the coiled tubing over the gooseneck.